Stripping is a process used to remove .volatile components from water. The basic concept is to bring the contaminated water into intimate contact with a stripping gas, frequently air, so that the volatile components undergo a phase change from liquid to vapor and are carried away by the stripping gas. A number of interrelated design factors affect the stripping efficiency: the Henry's law coefficient of the volatile component, the stripping gas:water volume flow ratio, the contact time and the mass transfer rate. The gas:water volume ratio used to remove the component from water depends on the volatility of the component to be removed, its concentration in the feed water and the physical attributes under which the contact is carried out. It is typically in the range 50:1-500:1 or more. The contaminant gas is, therefore, diluted by this amount when it is transferred from the water to the gas. When other factors are constant, a high gas:water volume ratio provides a high percentage of gas removal from the water, but creates large volumes of stripping gas with dilute concentrations of the stripped gas. A low gas:water volume ratio may provide insufficient dilution of the stripped gas to maintain a good driving force for mass transfer. Under optimum conditions, transfer of the component from the water to the stripping gas can be very efficient and removal rates up to 99.99% can be achieved.
The principal disadvantage of gas stripping is the air pollution that is caused when the waste gas is discharged. Various treatments have been proposed for this exhaust stripping gas. U.S. Pat. No. 4,892,664 describes an air-stripping system followed by catalytic oxidation of the contaminated air. U.S. Pat. No. 4,857,198 describes air-stripping in combination with a mixed carbon adsorption/biological treatment for the waste air. U.S. Pat. No. 4,517,094 also briefly mentions combinations of air stripping and carbon adsorption. Stripping can be used to remove or recover both volatile organic compounds and dissolved inorganic gases. Following are examples of stripping operations with inorganic gases.
Stripping Hydrogen Sulfide
Stripping can be used in the recovery of hydrogen sulfide. Hydrogen sulfide, which is slightly soluble in water, may be found in groundwater in dilute concentrations (approximately 1 ppm). Stripping hydrogen sulfide from groundwater allows it to be used as drinking water. The hydrogen sulfide concentrated stream is discharged to a catalytic process for further treatment.
Refinery and coking operations, however, often produce streams with higher concentrations of hydrogen sulfide (100 ppm). Such streams are also stripped, but the concentrated hydrogen sulfide discharge stream is sent to an incinerator.
Stripping Bromine
Stripping is also commonly employed in a bromine recovery process, which has gained significant importance due to economics and environmental pressures. The recovery process either involves deriving bromine from its natural state of occurrence or recovering bromine from bromine waste. Bromine occurs as bromide in seawater (0.188% Br), in the mother liquor from salt wells of Michigan, Ohio, West Virginia, and Arkansas, and in the potassium deposits of France and Germany. In the U.S. nearly all bromine is derived from the natural brines. Bromine containing waste is generated by bromination reaction of organic compounds in the production of numerous specialty chemicals, including pharmaceuticals, flame retardants, and agrochemicals. Bromination reactions generally produce the desired bromine containing end product and large quantities of bromine containing byproduct such as hydrogen bromide and sodium bromide. This large volume of waste must either enter the waste stream or be recovered for other uses.
The process begins with the oxidation of the bromine-containing solution by chlorine or oxygen. The oxidized solution is subjected to a steaming-out process, where it is steam stripped to sweep out the bromine vapors. The steam-containing bromine vapors are condensed and collected in a decanter, where bromine is recovered by phase separation. Bromine recovery from streams with lower concentration, i.e. where the bromine concentration is less than 200 ppm, is effected by air stripping instead of steam stripping after oxidation.
Stripping Hydrogen Cyanide
Stripping is also used to separate hydrogen cyanide from water. Cyanide and its various salts, including sodium cyanide, are frequently used in metal electroplating operations. The rinse water produced by such operations is contaminated with cyanide and its salts, which are in unstrippable form. Acidifying the contaminated water, however, produces hydrogen cyanide which is strippable. After stripping, the water stream is recycled and the stripped hydrogen cyanide may be incinerated or recycled.
Stripping Sulfur Dioxide
Stripping is also used in sulfur dioxide recovery process. Sulfur dioxide contaminated streams are commonly produced in sulfuric acid manufacturing operations as blowdown water. These streams are stripped and the stripped sulfur dioxide is recycled.
Stripping Carbon Dioxide, Ammonia, Hydrazine, or Oxygen
Stripping is also used to separate carbon dioxide, ammonia, hydrazine, or oxygen from water. Numerous industrial processes produce water streams containing carbon dioxide, which may be present either as a contaminant or as a desired compound to be recycled. Ammonia is produced in fertilizer production, coal gasification and refinery operations. In fertilizer production, the recovered ammonia is the desired end product. In coal gasification and refinery operations, ammonia is a contaminant and the recovered ammonia is sent to an incinerator. In boiler operations, hydrazine is used as an antioxidant and may be recovered by stripping if necessary. Excess oxygen, produced in boiler operations, is stripped from water using nitrogen as a preventive measure against corrosion.
That membranes have the capability to separate inorganic vapors and gases from other gases is known. For example, U.S. Pat. No. 4,608,060 describes the separation of hydrogen sulfide, sulfur dioxide and ammonia from such gases as methane, nitrogen, hydrogen and carbon dioxide using a composite membrane of polyethylene glycol and silicon rubber mixture on polysulfone. U.S. Pat. No. 4,963,165 describes the separation of carbon-dioxide from such gases as methane and nitrogen using composite membranes of polyamide-polyether block copolymer on a microporous support material.
Membranes have been used with stripping columns before to recover and reuse a significant fraction Of sensible and latent heat present in the gas/vapor overhead mixture from a stripping column. U.S. Pat. No. 4,444,571 describes the use of a semipermeable membrane, which is more permeable to steam, the stripping agent, than the contaminant gas. The permeate, primarily consisting of the stripping agent, is fed back directly into the stripper for reuse, which avoids the condensation of the overhead mixture performed in conventional stripping operations. The permeate, however, still contains a significant concentration of the contaminant gas. Reuse of this permeate in stripping operations greatly reduces the driving force of mass transfer of the component from the water to the gas. Hence, the stripper efficiency decreases noticeably.